Compact, efficient, and low-cost laser sources in the visible spectral range have long been desired for a variety of applications. These applications include laser-based projection displays, optical storage, bio-analytical instrumentation, semiconductor inspection and spectroscopy. Semiconductor lasers, which provide a low-cost, compact, and efficient platform, rely on material systems such as InGaAlP that lase most efficiently in the near-infrared spectral region. Efficient operation down to ˜650 nm (red color) can be achieved without serious technological challenges and some semiconductor laser designs can be extended down to ˜635 nm with however decreasing efficiency and reliability. On the shorter wavelength side of the visible region, GaN systems have been developed in recent years and lasers in the violet (˜400 nm to ˜445 nm) spectral range have been commercialized. However, achieving wavelengths >460 nm in an efficient and reliable way represents a serious challenge. Thus, the majority of the visible spectrum (i.e., from ˜460 nm blue to ˜635 nm red) does not currently have an efficient semiconductor laser solution.
Of these colors (wavelengths), the absence of green is perhaps the most notable since this color corresponds to the peak sensitivity of the human eye. Indeed, no direct solution for a green semiconductor laser is currently commercially available. The indirect solution, commercialized since the 1990s, has been based on nonlinear frequency doubling (also known as second-harmonic generation, or SHG) of neodymium (Nd)-based solid-state lasers, such as Nd:Y3Al5O12 (Nd:YAG) or Nd:YVO4. These solid-state gain materials can be pumped by infrared semiconductor lasers (e.g., at ˜808 nm or ˜880 nm) and produce laser radiation at ˜1064 nm wavelength. This 1064 nm radiation can then be frequency doubled into the green 532 nm wavelength using nonlinear crystals such as Potassium Titanyl Phosphate (KTP) or Lithium Triborate (LBO). A similar technique can be used to obtain the blue color, e.g. 473 nm by frequency-doubling a 946 nm solid-state laser. Furthermore, the low-cost platform can be achieved by using so-called microchip technology, where the solid-state gain chip and non-linear crystal are bonded to form a monolithic laser cavity. A microchip implementation has been proposed by Mooradian (refer to U.S. Pat. No. 5,365,539).
However, the currently available microchip lasers lack the efficiency and flexibility required in many applications. The compactness and efficiency specifications are especially important in mobile, consumer-electronics applications such as handheld, battery-operated projectors. These limitations in existing platforms are mainly due to the frequency conversion inefficiency of conventional nonlinear materials such as KTP. In order to obtain high-efficiency green color output from a KTP-based microchip laser, one has to provide a significant power margin for the fundamental Infrared laser, which imposes thermal, size, and cost limitations on the overall laser system design. Furthermore, traditional bulk nonlinear materials such as KTP are restricted as to their scope of frequency conversion. For example, KTP is used for frequency doubling into the green color but cannot be practically used for frequency doubling into the blue color, so one has to search for different nonlinear materials with their own limitations in efficiency, reliability, and cost. In addition, KTP crystals, which are widely used in low-cost green laser pointers, suffer from reliability problem called “gray-tracking,” which is associated with light-induced absorption in the crystal.
Laurel (refer to U.S. Pat. No. 6,259,711) proposed that many of such limitations can be overcome by the use of periodically poled nonlinear crystals. These crystals can be engineered to provide high nonlinearity for the desired conversion wavelength. Therefore, such a laser design implemented in a microchip architecture, could address many of the restrictions associated with conventional bulk nonlinear materials.
However, embodiments of that invention suffer from significant limitations, which, to our knowledge, have prevented commercialization of this platform, and, to this day, visible wavelength microchip lasers continue to rely on bulk nonlinear materials such as KTP and KNbO3, the latter material being used to produce the blue color (see, e.g., published PCT application WO 2005/036703). The origin of such limitations lies in the choice of periodically poled nonlinear crystals proposed in Laurel's invention, i.e. KTiOPO4 (KTP), LiNbO3 (LN), and LiTaO3 (LT). These materials possess high nonlinearity and can be readily poled into periodic structures for frequency doubling. However, the practical use of these materials is very limited. Like bulk KTP, periodically poled KTP can only perform well at low power levels (a few milliwatts or possibly even tens of milliwatts in the visible) but suffers from induced absorption (“gray tracking”) at higher power levels. In addition, KTP crystal production is not easily scalable to high volume quantities at low cost as is required by some applications such as consumer-electronics displays. LiNbO3 and LiTaO3 are scalable to high-volume production and can be readily periodically poled, but suffer from visible-light-induced degradation (“photo-refractive damage”) that makes it impossible to use these crystals to produce even milliwatts of visible light without severe degradation. The photo-refractive damage can be reduced at elevated temperatures (>150° C.). However, this requires using ovens for maintaining the nonlinear crystals at a high-temperature. Such ovens are incompatible with a low-cost, efficient laser fabrication, especially in the microchip geometry. Thus, the laser designs described by Laurel, cannot be implemented in a high-power, low-cost, compact, and efficient architecture. Similarly, Brown (refer to published patent application US 2005/0063441), proposed designs for compact laser packages, which would appear to be suitable for low-cost applications. However, the Brown teaching is still centered on conventional nonlinear materials such as KTP and LBO. The possible use of PPLN (periodically poled lithium niobate) and PPKTP (periodically poled potassium titanyl phosphate) is mentioned but it is not taught how one can overcome the limitations of these crystals, especially their aforementioned reliability limitations.
It is known that congruent LiNbO3 and LiTaO3 suffer from photo-refractive damage due to visible light, and several ways to overcome this problem have been proposed. The high-temperature operation, mentioned above, partially solves the problem, but is not suitable for most applications. Another proposed solution is doping the congruent material during the crystal growth to suppress photo-refractive damage mechanisms. Growing bulk crystals with a high degree of stoichiometry has been proposed as another method to suppress photo-refractive damage. However, none of the prior art workers have taught a means of achieving a high output power, stable ambient-temperature-operable frequency doubled laser in the microchip architecture for producing high-efficiency green and blue light in a low-cost design suitable for mass manufacturing.
Recently, several approaches were proposed to overcome reliability constraints for visible laser sources based on periodically poled materials. In particular, the use of periodically poled lithium niobate and lithium tantalate using materials with MgO, ZnO, or other dopants to overcome reliability problems was suggested. However, most of architectures for such visible laser sources were not simple enough as compared with the simple microchip architecture.
In particular, Corning, Inc. (refer to US patent application by Bhatia et al., 2008/0089373) proposed to use the nonlinear waveguide geometry, based on MgO-doped periodically poled lithium niobate to efficiently frequency double a near-infrared semiconductor laser. However, this architecture is quite expensive since it relies on the use of high-cost components (special “DBR” semiconductor laser with phase control, waveguide as opposed to bulk periodically poled MgO-doped material), and the high-cost alignment steps including sub-micron alignment of two single-mode optical waveguides, which have to be kept in alignment over the lifetime of the laser source.
Osram Semiconductors (refer to US patent application by Kuehnelt et al., US2007/0081564) proposed to use periodically poled, MgO-doped lithium niobate for intracavity frequency doubling of an optically pumped, surface-emitting semiconductor laser. While it eliminates stringent tolerances associated with single-mode waveguides, this architecture is still at significant disadvantage as compared to the microchip architecture due to its rather low optical gain (and thus limited overall efficiency) and the need to assemble and align multiple discrete optical components.
In short, known technical approaches cannot provide a reliable, cost-effective, and compact frequency-converted laser for high-efficiency visible (in particular, green-color) output.